Improved H2 uptake capacity of transition metal doped benzene by boron substitution

Abstract

The effect of boron substitution on hydrogen storage capacity of transition metal (TM) doped benzene is studied using density functional theory and the second order Møller–Plesset method with aug-cc-pVDZ basis set. Out of the six carbon atoms in a benzene ring, two are substituted by boron atoms. The structures considered here are C₄B₂H₆TM (TM = Sc, Ti, V). Four, four and three H₂ molecules can be adsorbed on unsubstituted C₆H₆Sc, C₆H₆Ti and C₆H₆V complexes, respectively, whereas upon boron substitution one additional H₂ molecule gets adsorbed on each of these complexes. The H₂ uptake capacity of C₄B₂H₆Sc, C₄B₂H₆Ti and C₄B₂H₆V obtained is 7.71, 7.54 and 5.99 wt%, respectively. Gibbs free energy corrected adsorption energies show that H₂ adsorption on C₄B₂H₆Sc is energetically unfavorable whereas it is favorable on C₄B₂H₆Ti and C₄B₂H₆V at ambient conditions. Various interaction energies for the H₂ adsorbed complexes are obtained using a many-body analysis technique. The H₂ desorption temperature for boron substituted TM doped benzene is lower than that for TM doped benzene for all the three systems. Molecular dynamics simulations show that loosely bonded H₂ molecules in C₄B₂H₆Sc(5H₂) and C₄B₂H₆Ti(5H₂) complexes fly away during the simulation, thereby showing lower H₂ uptake capacity of these complexes than that obtained by electronic structure calculations.